156 Bulletin of the Geological Society of Denmark Bioerosional evidence of rocky palaeoshores in the Neogene of Portugal: environmental and stratigraphical significance CARLOS MARQUES DA SILVA, MÁRIO CACHÃO, JORDI MARTINELL & ROSA DOMÈNECH Silva, C. M. da, Cachão, M., Martinell, J. & Domènech, R.: Bioerosional evi- dence of rocky palaeoshores in the Neogene of Portugal: environmental and stratigraphical significance. Bulletin of the Geological Society of Denmark, Vol. 45, pp. 156–160. Copenhagen 1999–01–30. Occurrences of Neogene rocky shores in Portugal have been identified by help of bioerosion trace fossils. Ichnogenera Gastrochaenolites, Entobia, Caulostrepsis, Maeandropolydora and Trypanites are generally present. The importance of bioeroded surfaces for field recognition of wave-cut platforms, hard substrate marine-flooding surfaces (transgressive surfaces), and of major interruptions (hia- tuses) in the stratigraphic record is stressed. Key words: Bioerosion, rocky palaeoshores, Neogene, Miocene, Pliocene, Portu- gal. C. M. da Silva & M. Cachão, Departamento de Geologia, Universidade de Lisboa. Rua da Escola Politécnica, 58. P-1294 Lisboa Codex. Portugal. Museu Nacional de História Natural - Grupo Paleo, Centro de Geologia da Universidade de Lisboa. J. Martinell & R. Domènech, Departament d’Estratigrafia i Paleontologia, Facultat de Geologia, Universitat de Barcelona, E-08071 Barcelona. Spain. 15 September 1998. As exposed rock surfaces associated with a low or zero rate of sedimentation, rocky shores provide a very favorable environment for bioeroding organisms. Rocky shores are also remarkable from a geological point of view. They represent a unique marine sedi- mentary environment, one in which erosional proc- esses dominate (rather than depositional processes) and where wave-cut platforms are being formed. Therefore the identification of rocky shores in the geological record is very important, for they repre- sent major flooding surfaces and provide us with cru- cial information about palaeoshorelines and ancient sea levels. Bioerosion structures play a major role in their recognition. Bioerosional evidence allows us to reconstruct the evolution of the marine hard substrate biota, which is far less well known than that of soft bottom commu- nities. Rocky-shore communities, which are known in strata as old as the Lower Ordovician, are com- posed of three main types of organisms: borers, encrusters and clingers (Johnson 1988, Bromley 1994), all of them recognised in the fossil record al- most exclusively by their trace fossils. This contribution briefly describes and comments on the Portuguese occurrences of bioerosion structures associated with rocky shores – mineralised, lithified surfaces – that were active during Neogene times. An overview of the same kind of Neogene substrates lo- cated on the Mediterranean side of the Iberian penin- sula and southeastern France may be found in Gibert, Martinell & Domènech (1998). Bioerosion studies in Portugal The oldest reference to Portuguese Neogene trace fos- sils in lithified substrates goes back to Choffat who, in 1903-4, in the Nossa Senhora da Vitória site, rec- ognised the occurrence of “…cailloux calcaires, ar- rondis [of Hetangian age], ayant de nombreux trous de coquilles perforantes [of Pliocene age]” (Choffat, 1903-4: 112). Much later, other authors referred to the existence of perforations (identified as the work of the bivalve genus Pholas) in the contact surfaces of Jurassic/Neogene formations in Algarve, southern Portugal (Pais 1982, Antunes 1984). In Portugal, the palaeoecological and palaeoenvi- ronmental significance of bioerosion structures, namely as indicators of rocky shores, has only lately been recognised and utilised (Silva, Cachão, Martinell